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How are electronic instruments and other equipment powered in a glider?
Radios and other electronic devices must be battery-powered. What sort of battery - and does its capacity impose a limit on flight duration, or can they be recharged in flight with a turbine?

Gliders generally use "Gel Cell" or Sealed Lead Acid rechargeable batteries, a little bigger than a block of butter, that will run designed-for-glider low wattage radios, and sometimes transponders, and FLARM systems (a kind of TCAS) and various other gadgets, and are good for several hours of operation (the total current draw of all the services might be under 1A when not transmitting on the radio, so a 5 AmpH Gel Cell 12v battery might be plenty unless you are talking all the time). Enough for normal glider flights and tasks, and if necessary you can hook up a second one if expect to be out for more than 4-5 hours.
Some newer batteries of the more exotic Lithium chemistry are starting to come on the scene, but the thing is the gel cells are light enough and are effective and trouble free, and don't require an STC (Supplemental Type Certificate; basically a 3rd party field modification), so the switch away from gel cells will be slow.
Recharging in flight with a turbine would be a real L/D killer so it's out of the question.  Much better just to install provision for a second battery for long flights.

Nice answer. I've also seen small solar panels on the glareshield or turtleback. They help the lead-acid battery last longer in flight and save the hassle of removing it from the aircraft to recharge between flights.

Thanks. Can you expand "STC"?

Block of butter can come in various sizes. Can you use a better example?

Say, roughly, 3" wide, 6" long, 5" high.  Something you can hold in your hand like a brick.  At least the ones in my club's gliders.

Simplified: STC = supplementära type certificate. Any change done to the design of an aircraft has to have paper work. The STC is the paper work allowing you to do specific changes, say changing the type of batteries to a type not originals part of the plans. The STC has to be created by someone and approved by the relevant authorities (ie casting time and money).


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Are larger aircraft inherently better-suited to long-distance flights?
Restricting this to larger passenger aircraft, and apart from the greater range made possible by larger fuel tanks, are larger aircraft better-suited to long-distance flights simply in virtue of their size?

Aerodynamically? Financially? Comfort-wise? Speed-wise? Please try to clarify

@J.Hougaard Neither ? OP clearly ask if range scale with size.

@J.Hougaard "Financially"? How is that a property of an aircraft? Speed: as you no doubt already know, the speed of passenger aircraft is mainly determined by the speed of sound with nearly all jets cruising at roughly the same speed regardless of size. If however you know something relevant about the effects of scaling on aerodynamics or passenger comfort, by all means add that to an answer.

@DanieleProcida It's quite an obvious property from a business (airline) point of view.

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text:
What is the physical scale of the turbulence that affects large passenger planes?
What magnitude of displacement of air in turbulence conditions is large enough to affect (i.e. be noticeable in) an airliner?
Are there any minimum limits, so that even fiercely turbulent air below a certain displacement is not noticeable?

Noticeable is a wide range.  Also wing loading plays a role, as it tends to reduce the effect on the aircraft to vertical displacements.  Also there are many classifications of turbulence. Finally, there is a spectral component, where lower frequency turbulence (larger area up and down drafts, for example) and higher frequency (chop, for example) have different effects on passengers and crew. Yet wake turbulence, may create roll moments, which can be more upsetting to a flight. So displacement is a factor, but frequency, direction and other attributes impact flight comfort and safety.

This seems very similar to this question

@mongo I realise that displacement is only one factor, but it is the one I'm asking about... However, could you expand on your mention of spectral components and their effect on passengers and crew as (part of) an answer to https://aviation.stackexchange.com/questions/42527/how-do-subjective-experiences-in-the-airliner-cabin-correlate-with-meteorologica?s=16|0.3687, as that is exactly what that question is about?

Summary
There are in effect no minimum levels of displacement that will not have noticeable effects. Even very small displacements, of an appropriate period and spectrum, can affect a large aircraft. Scale alone is not a limiting factor.
In more detail
Question 2: Are there any minimum limits, so that even fiercely turbulent air below a certain displacement is not noticeable?
The title addresses "large passenger planes" and not passengers, or passengers with vestibular anomalies, etc.
Generally mountain wave is considered "fiercely turbulent" as the updrafts and downdrafts can exceed the climb and descent capability of an aircraft.  However, the "frequency component" of the mountain wave can be very low, at times creating no human perception of motion. Yet the turbulence causes loss of control by virtue of exceeding the aircraft's capabilities of counteracting an upsetting force.
So in this instance, it is possible to have an extreme situation, and actually loss of control, by turbulence which is imperceptible by a human without instrumentation.
Question #1: What magnitude of displacement of air in turbulence conditions is large enough to affect (i.e. be noticeable in) an airliner?
The aircraft, and it's contents (including people) act as a system. The system can dampen the effects of some forces and cause amplification of other forces.
A tiny, repeated push to a child on a swing, can result in very large displacements of the swing, and is a good example of a form of resonance or amplification of the effects of a small push.  The pushes have to have the right periodicity, matching the resonance of the swing to be effective.
The above is an example of a resonant system, but we also have around us systems which dampen energy, such as the suspension on a car.  Driving a car over a bump in the road might create a very mild sensation in the car, but if we were riding on a solid wheel hitting the same bump at the same speed the resulting bump could be injurious.
So to only ask about displacement is akin to only asking about sound pressure of music. The frequency (notes) help create the music, and the periodic occurrence of the frequency energy also helps create the music (like beat). A drum strike can be very loud or a triangle very soft, but they are both noticeable in music, in part due to our perceptual training, but also due to the human auditory system.
So for an aircraft, a displacement at one frequency may have essentially no impact on the flight path, due to the systemic response of the aircraft. Furthermore that displacement may or may not be detectable by a human riding in that aircraft.
 To answer your broad, but limited to displacement question, becomes impossible to do so accurately if you choose to limit the dimensionality of the system you are seeking analysis of. (reference your comment)

Thanks, this is the kind of answer I was looking for. I added a summary at the top, just to make it a bit clearer - please feel free to amend it further.

The"tiny push at resonant frequency" trick doesn't work if the child is activity trying not to swing, though; and planes have control surfaces.

If the child is actively trying not to swing, then they are changing the "system" and the tiny push does not work.  Not sure what you intended to mean about control surfaces.

On the summary added, period and spectrum are the same. They are both temporal domain.

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Was Ludwig Wittgenstein's aircraft propeller ever built?
Ludwig Wittgenstein designed (and received a patent for, at the remarkable age of 22) an aeronautical propeller.
Wikipedia describes it:

Wittgenstein’s design required air and gas to be forced along the propeller arms to combustion chambers on the end of each blade, where it was then compressed by the centrifugal force exerted by the revolving arms and ignited.

and adds:

Propellers of the time were typically wood, whereas modern blades are made from pressed steel laminates as separate halves, which are then welded together. This gives the blade a hollow interior, and therefore creates an ideal pathway for the air and gas.

I've found some plans for this design, and some discussion of it, but it's not clear to me whether a working model was ever built, or attempted. I'd love to know whether it actually offered any advantage, or even whether it could be made to work with a reasonable degree of success.

Ian Lemco of the Royal Society published a note on this in 2007. He described the invention more as a jet engine than anything, as it featured a central compressor feeding tip-jet nozzles. Burner position varied during development of the design, from central to blade tip. A patent dating from 1910 shows jet combustion at the tip. One central compressor was of the centrifugal type. Thus, it is seen in some ways as foreshadowing the Whittle jet engine.
Lemco found sources to be scarce, mostly anecdotal, and unreliable. Certainly Wittgenstein constructed various experimental devices and tested them, both in the lab and on a railway track. Some may have been rockets. One source remarks in passing on the engine that he constructed, but the reliability and context are far from clear. There is no confirmation that he ever built a complete jet engine.
It seems to have been fundamentally workable, as compressor-driven tipjet rotors were indeed later flown, but it was probably beyond the technology of Wittgenstein's day.
Ian Lemco; "Wittgenstein's Aeronautical Investigation", Notes and Records of the Royal Society of London, Vol. 61, No. 1 (Jan. 22, 2007), pp. 39-51. Stable URL: http://www.jstor.org/stable/20462605

In a way, yes.
The Doblhoff WNF 342 was a helicopter with tip jets, flown first in 1943, with rotor propulsion not unlike Wittgenstein's idea. Wikipedia describes it:

The conventional piston engine drove both a small propeller (to provide airflow across a rudder) and an air compressor to provide air (subsequently mixed with fuel) through the rotor head and hollow rotor blades to combustion chambers at the rotor tips.

The compressor helped to get the blades moving. This, however, was not used on the propeller, but on the rotor. Another design which used a rotor with tip jets is the Fairey Rotodyne which flew first in 1957. Again Wikipedia:

the Rotodyne featured a tip-jet-powered rotor that burned a mixture of fuel and compressed air bled from two wing-mounted Napier Eland turboprops.

The noise from the tip jets proved to be its undoing, however, since it could not attract orders because of concerns over the noise.
In the US, tip-ramjet powered micro helicopters were successfully developed in the late Forties and Fifties (see XH-20, XH-26 and YH-32) but all suffered from high noise and low fuel efficiency, so the concept never went beyond the prototype stage. They were different from Wittgenstein's concept, however, because they did not use centrifugal acceleration to compress their working medium but relied solely on ram pressure which is low at the subsonic speed of helicopter blades. This low pressure resulted in low efficiency of the ramjets and in their high fuel consumption.

@qqjkztd: Thank you for the hint; I've not known about the DF1 so far. Using the rocket fuel of the Me-163 makes it – how should I say – interesting. Availability of the fuel is probably also an issue.

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What is the relation between the EASA Light Aircraft Pilot Licences and the Private/Sailplane/Balloon Pilot Licences?
The EASA licence and ratings requirements pages list:

PPL(A) - for aeroplanes
PPL(H) - for helicopters
SPL - for gliders
BPL - for balloons

which match:

LAPL(A) - for aeroplanes
LAPL(H) - for helicopters
LAPL(S) - for gliders
LAPL(B) - for balloons

It appears that you can upgrade an LAPL to its matching licence; for example, an LAPL(A) to a PPL(A) by taking further instruction and tests (except in the case of the sailplane licence, which seems not to require this, and the airship licence PPL(As), for which their exists no LAPL counterpart).
It looks like the LAPLs are generally easier to obtain (for example, requiring fewer flying hours).
What's the purpose of this apparent duplication of licensing? Are the LAPLs intended to be a pathway to the other licenses, or a destination in themselves?

Since you're discussing EASA and the UK CAA, it might be worth keeping in mind the issue of whether the UK will remain in EASA post-Brexit. I've heard, but not verified, that recent policy of the UK has been that by leaving the EU, they will also be leaving EASA. This of course doesn't mean that the UK can't keep something very similar to the EASA LAPL in place, but absent specific bilateral/multilateral agreements, if the UK leaves EASA, then any UK LAPL will not confer the rights and privileges of an EASA LAPL when in an EASA country, even if the requirements of the two are identical.

LAPL (Light Aircraft Pilot License) can be a pathway to a PPL (Privat Pilot License) but can also be just kept by it's own, as it's somewhat cheaper to get and maintain than a PPL.
LAPL has stricter limitations than the PPL, for example they are only valid in EASA countries and not in all ICAO states. Also, LAPL is limited to a maximum take-off weight (MTOW) of 2000kg and does not have the options of upgrading to IFR or similar (Night VFR is an option though) and a maximum of 4 people on board (3 passengers + you).
Therefore: If you are fine with the mentioned limits, you're good to go for an LAPL. If at some point you want to upgrade to PPL, thats not a big issue as it just requires the additional training hours (theory is the same for LAPL and PPL).

The first group you list (PPL, SPL, BPL) are ICAO-level as also mentioned in the answer by rauberdaniel, while the LAPLs are EASA only.
One significant difference is that the ICAO-level ones require a class 2 medical, while the LAPL licenses require the less demanding LAPL medical. For some people, this makes all the difference - at the cost of being restricted to flying in EASA-land. Also notice that a class 2 medical, when it expires becomes a LAPL medical (which is "contained" in the class 1 and 2 medicals) until that one expires, too.
Another difference in the gliding corner of the field is that the SPL allows to receive compensation for the flight (getting paid to fly, basically), while the LAPL(S) does not.

The CPL (Commercial Pilot License) allows for compensation for the flight, the PPL does not (except Flight Instructor / splitting the costs evenly).
SPL is Glider License, that's something completely differnt.

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What airport fees are charged to airlines in the case of operations following an unscheduled landing?
A plane may need make an unscheduled landing in the case of a diversion due to mechanical difficulties, unexpected weather, a medical emergency or other reasons (this could even be at an airport with which the operator has no prior agreements).
Airport operators agree various fees with the airlines that use them for their scheduled operations:

runway/landing fees
fees for stairs, buses to the terminal building, or airbridges
passenger charges, should the passengers need to disembark
parking fees
...amongst others

Which of these charges are typically levied in the case of an unscheduled landing caused by unforeseen circumstances?

A while back I was on my way to Frankfurt on Delta for a business meeting when a frail passenger (who quite honestly should have never been allowed to board in the first place) passed away before getting to the midpoint of the trip. The captain turned the aircraft around and landed at Gander International in Canada, where we spent the night before continuing on our way.
Because I've seen this in the news once in a while, I decided to check the airport's web site to see what is their fee structure. Lo and behold, they publish a comprehensive list of fees that covers everything related to an occurrence of this type.
The fee structure even covers "air rage" incidents under the category of "Emergency" and is charged at so-many-dollars per 1000 kg!

Emergency
  Including medical, mechanical, air rage, etc.
  $12.57 per 1000kg

That is simply the land fee, it doesn't say if other fees the OP asked about are chargeable.

The document that Juan Jimenez referred to mentions landing fees, terminal fees, and parking fees. The fact that it mentions a special rate for emergency landings suggests that the other fees (parking etc) apply whatever the case.

@DanieleProcida Indeed they do, I never suggested otherwise, but my answer is directed towards the situation presented in the question, and not all operational situations.


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Do ailerons on opposite wings move together?
It seems nicely symmetrical that opposite ailerons would be locked togther, so that when the left wing's ailerons move up, those on the right wing would move down, by the same amount and at the same speed.
Is this in fact generally the case on aeroplanes? Or can they be moved independently, or at different rates or to different positions (and what advantage would that offer)?
Finally, are there any aeroplanes on which they can be moved in the same direction?

That's pretty much what they do so I'm not really sure what you are asking.  And if they moved in the same direction they would not functionally be considered ailerons, they would be either flaps or spoilers.

@MichaelHall: There is also a combination, called flaperons. They move together with the flaps and - by means of a mechanical mixer - work as ailerons on top of that.

@MichaelHall It seems that that is what they would do, but I understood that in fact it's not what they always do, hence the question.

On most general aviation airplanes, and other cable-controlled aircraft, they move together and for that matter are cross linked to the same physical cable. You can see this nicely on the control diagram for the B-24

(source)
Some ailerons are designed to counteract adverse yaw and may move such that they are asymmetrical to the flow over the wing.

1) Differential Ailerons: One aileron is raised a greater distance
  than the other aileron is lowered. The extra upward aileron movement
  produces more drag change than an increase in AOA on the downward
  aileron. This produces an increase in drag on the descending wing,
  which reduces adverse yaw.
2) Frise Ailerons: The aileron being raised pivots on an offset hinge.
  The leading edge of the aileron is now pushed into the airflow,
  creating drag and reducing adverse yaw. In this case, frise ailerons
  are using form drag to counter induced drag.

But not all aircraft contain such systems. Fly-by-wire systems are theoretically capable of deflecting any control surface as they see fit and may not move symmetrically but generally speaking their direction of movement is opposite. Some fly-by-wire aircraft even have split ailerons which may see parts of a give aileron deflect differently. This answer covers that quite nicely.
If you really want to go back, the 1912 Farman HF.20 had single action ailerons that only deflected downward and were pushed back into position by the airflow. I would say (if these count as ailerons) that they do in fact not move in tandem. There were some other early designs covered here that worked a bit differently than modern ailerons.

As for your second point, aircraft that have elevons, which serve as both the elevator and the aileron (mostly delta wing stuff like the Concorde and other high speed airframes), can move the surfaces in the same direction. Generally other surfaces are employed for the use case of needing to move both ailerons in the same direction (speed brakes).

Pictures of the HF.20 on the ground show both ailerons in line with the wing. You need to go back to earlier types like the Voisin-Delagrange to find one where both ailerons were hanging down, clearly indicating that they were free-floating in operation.

IIRC the A320 raises both ailerons to support the ground spoilers after landing.

Or can they be moved ... at different rates ... ?

I saw a power glider (maybe an SF-28 or an RF-5) where the ailerons moved with a different rate.
The reason was an effect whose German name translates to "negative turn effect", but I don't know the correct English word for it:
If you operate the ailerons to the right, the ailerons work in a way that the left wing produces more lift and therefore more air resistance than the right one.
Therefore operating the ailerons to the right will also have a similar effect as operating the rudder to the left. (For this reason the rudder has to be operated to the right together with the ailerons to compensate that effect.)
The power glider I have seen was built in a way that the left aileron only did a much smaller motion then the right one when putting the stick to the right. By doing so, the right aileron worked like an air brake so the air resistance of both wings was nearly equal and there was no "negative turn effect".

Finally, are there any aeroplanes on which they can be moved in the same direction?

There are airplanes that use "flapperons":
This means that the same part is used as flap and as aileron.
Being used as flaps, the flapperons move in the same direction; being used as ailerons, they move in different directions.

The basic principle of ailerons is that the aileron on the down-going wing will deflect up, and the aileron on the up-going wing will deflect down. But the movement usually isn't equal on both sides.
Moving the aileron down increases the wings camber and therefore lift, which also comes with drag. This drag produces a yawing effect opposite to the desired turning direction. This is known as adverse aileron yaw, and needs to be overcome by coordinated use of the rudder when banking.
There are a few different ways of reducing this adverse yaw, the most common of which is known as differential ailerons. This is where the aileron that deflects up has a greater range of movement than the one that deflects down, reducing the drag on that side for the same banking force.
On a side note, airliners don't use ailerons at all at high speeds, because doing so will twist the wing. Spoilers are used instead for roll control in cruise as they are closer to the wing root, thus providing less twist. They operate only by deflecting up on the one side.

The airspeeds required to cause significant wing twist from aileron deflection in airliners are generally well above said airliners' never-exceed speeds.

@Sean: Depends on what is significant to you. Boeings, Illyushins and most other airliners use high speed ailerons in the gap between inner and outer flaps, so the outboard ailerons need only be used in slow flight. Only Airbus doesn't. At cruise aileron effectivity is markedly reduced because of wing twist. Only a reversal speed below v$_{NE}$ would make certification impossible.


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Are passengers more likely to see other airliners from a window seat on the right or on the left?
From the window of an airline, one is more likely to see a plane travelling in the opposite direction, than one flying in the same direction.
Are planes travelling in the opposite direction more likely to be found on the right or the left? That is, are opposite airways given a left/right arrangement?

There is no general rule for this. It will vary based on weather, prevailing winds, terrain, how busy the airport is, whether there's a conflict in the area, etc.

A partial answer to my own question...
The jet-streams over the Atlantic mean that traffic flying west from northern Europe to north America will seek a path further to the north than traffic coming in the opposite direction.
So there at least, there is more likely to be passing traffic on your left than on your right. That's an arrangement imposed by high-altitude wind patterns rather than by fiat of course. Moreover, the distances between the tracks mean that while the traffic might be there, there's no guarantee of actually seeing it.


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What are the aerodynamic implications of different conditions with the same Mach number?
At a particular Mach number, a fluid behaves similarly, but many other conditions could be different.
For example, an aircraft flying at Mach .85 at a low altitude will be in denser, warmer air, and actually flying faster, than the same aircraft flying at Mach .85 at a high altitude.
What aerodynamic differences (if any) will the aircraft experience between the two conditions?
As far as flight aerodynamics is concerned, which of Mach number, true airspeed and indicated airspeed are most significant, and when?

The Mach number only indicates the behavior of compressibility effects, but does not provide any indication about other effects like inertia and viscosity (read turbulence) or heat transfer.
For high subsonic and supersonic aerodynamics one should typically compare both the Mach number (compressibility) and the Reynolds number (turbulence).
So at the same Mach number but different densities, the Reynolds number will be very different and therefore the turbulence effects will differ highly. Only the compressibility effects will be similar (shock waves).
Note that at low subsonic speeds (Mach<0.3), the compressibility effects become negligible and one can get away by only comparing the Reynolds number.
And at hypersonic speeds (Mach~>5) more similarity parameters are needed.
More information about dimensionless numbers in fluid mechanics on Wikipedia.

There are 2 excellent graphs in the question How does maximum speed vary with altitude?
For flight aerodynamics, we can set aside TAS, which is useful calculating time from point A to B, but not the aerodynamic effects.
That leaves us with IAS and Mach number.  At lower altitudes, the Never Exceed Speed has a Mach number less than 1, so it is expressed in IAS. Mach number of a given IAS increases with altitude, there for Mach is limiting "never exceed speed" at higher altitudes.
Temperature slightly increases speed of sound, so in warmer air Mach number will be slightly lower for a given IAS.
So the lower flying aircraft in warmer air at Mach .85 (temperature factor much less significant) will have a higher IAS than the one at higher altitude, there for experiencing stronger aerodynamic effects.
The graphs speak volumes in the linked question.

"For flight aerodynamics, we can set aside TAS, which is useful calculating time from point A to B, but not the aerodynamic effects." - that's true of ground speed, but surely not of true airspeed.

@Daniele Procida thanks for having me go back and double check this. Yes, True airspeed is the actual speed of the aircraft relative to the air mass (excluding wind direction and speed because the plane moves with the airmass).  But at higher altitudes the plane has to go faster to "feel" the same IAS, therefore TAS will be greater than IAS at higher altitudes.  Notice TAS is linked to MACH, which is why IAS gives higher MACH numbers at higher altitudes.

Never exceed speed actually depends on TAS (too)! There are three effects that limit maximum speed: pressures/forces, which depend on dynamic pressure, so IAS, transsonic flow separation, which depends on Mach number, and flutter, which depends on true airspeed. For aircraft that don't get into the transsonic region, but do get to high altitudes, the Vne is usually tabelated as descreasing with altitude due to flutter.


text:
How was the 747 in the China Airlines Flight 006 affected by the wing damage it sustained?
The passengers on China Airlines Flight 006 had a very memorable flight when the crew lost control and the aeroplane went into a high-speed dive.
Various pieces of the aircraft were torn off. The plane was repaired and returned to service, with its wings bent permanently upwards by 5cm.
How did this permanent damage affect it afterwards? Did it fly or perform noticeably differently? Did it require a special inspection regime for the rest of its operating life?

If a dimension has permanently changed, it means the metal has "yielded", or stretched beyond its elastic limit (like when you over stretch a spring and it's longer than originally, and cantilever metal structures are big springs).  The part that's yielded isn't as strong as before.  Perhaps they did a "geometry shoot" where the overall dimensions are measured and the deformation was still within specification and decided it was ok as is, but I would have expected the yielded components to have been replaced.  We don't know the details of the repair so there's no way to know for sure.

The additional dihedral probably made it more stable...

... and jauntier.

@JohnK Aluminium alloys are not necessarily weaker after stretching, as may be found in this doc

Wings flex quite a bit in flight, much more than 5cm, so I doubt that it had any special handling characteristics.

The linked article said the wings were bent upwards after the accident. It does not say that the damage was not repaired. The actual report says "the set was within the manufacturer's allowable tolerances" but again does not say if the damage was repaired.

@user71659 If something is "permanently bent" I think that implies that it stayed that way.

@DanieleProcida So they replaced the whole wing...

It also depends where they measured those 5cm...

@DanieleProcida  Actually mulling it over I would think that on a plane that big that a 5cm deviation from datum at the wing tip (it would be based on the airplane on jacks and fully defueled, allowing for the wing sagging under its own weight) was well within Boeing's geometry specs and it may have been that way when it left the factory. If it was in spec when they shot it, they're good to go aside from fixing other obvious damage.

In case anyone is wondering where the aircraft is now: https://www.747sp.com/747sp-production-list/22805-564/

As you know the aircraft manufacturers keep tracking their aircrafts because any accident of these would damage their reputation. All automatic data transmissions are traced by the manufacturer whether the operator has a tracking contract with the manufacturer or not. Should it had an asymmetrical behavior causing any dangerous consequences the manufacturer would have made what is necessary to ground it.
In the website you refer to, it is written « After repairs were made to the plane, it returned to service on 25 April 1985. It continued in service for nearly 12 years »:
I may assume this aircraft has landed in Europe, or in USA, should it have been dangerous, it would have been grounded.
As far economical performances are concerned, that’s secondary compared to safety, it is the operator decision to evaluate to make it fly or not if it becomes less economical.

Were automatic data transmissions a thing in the 1980s?

These are ACARS messages, automatic maintenance data started in the mid 80’s , but this aircraft resumed flying in 1985 for 12 years with the same operator  and even continued flying after that for some more years. Further Boeing has its own means to trace these problems


text:
Why are so many tail-strike incidents not verified until later inspection?
One often reads of a "suspected tail-strike" at take-off (for example suspected because a controller has seen something from the tower, or because a cabin crew member sitting at the back of the plane has felt a jolt or heard a noise).
What puzzles me is why this must remain merely "suspected" until an inspection.
As an example, a 737 suffered a tail strike that appears to have at least been considered a possibility by the crew. Apparently doubting that it actually had occurred, the flight crew continued to their destination; had there been a clear indication, they presumably would not have.
Aircraft are festooned with external sensors and measuring devices; I would expect aeroplanes susceptible to tail strikes to have sensors measuring movement of, deformation of or contact with a plane's skid plate, to provide a cockpit alert, so that there need not be any doubt.
Since this often doesn't appear to be the case, there must be good reasons why such systems are not usefully employed - what are they?

On one hand tail strikes are mitigated long before the need for a sensor. Aircraft makers generally address it from a design standpoint before the aircraft ever flys. If that is not enough control augmentation or tighter procedures are developed to prevent it from an operational level.
Ultimately such sensors do exist and do warn the crew of a tail strike they may just not yet be certified or installed on every airframe. For example it came in handy in this incident:

After the passengers had disembarked, the captain conducted an
  inspection of the tail skid of VZR and noticed some paint damage and
  scrape marks, however the cartridge containing the sensor for a
  tailstrike was still intact. This indicated that the tailskid had only
  just contacted the runway during the take-off.

Dave, in your second paragraph: how does the link relate to the statement? I visited the page and could not find anything about active sensors.

@bogl sorry bout that, I have updated it with a more relevant link and quote. I must have just thought that page had the info. It should be there now.

On aircraft not having a tail strike detector it relies on the captain to declare a tail strike that he encountered at takeoff or even at landing however, it is the responsibility of the captain to decide to continue the flight or not, nevertheless:

at landing the maintenance team should inspect the aircraft and if a noticeable damage is seen, they do a report
The technical crew(pilot...) is suppose to go around its aircraft and inspect any visible damage, this is done every time a technical crew is in charge of a flight
Slight tail strike doe not cause any damage and may not be felt by the crew, however when it is very slight it is felt only by the cabin crew at the rear of the aircraft, in this case they are supposed ...to inform the captain, and make a cabin report.

All modern aircraft do have a tail strike detectors B777, 787, all Airbus etc
The causes of tail strikes are numerous, even though long aircraft do have this tendency and are protected for such event  as the A321, whose fly by wire prevents this as possible, also it has a detector, and a memorized maintenance message.
A tail strike having caused damages should be repaired as recommended by the aircraft manufacturer otherwise it might be subject to a crash by flight control failure of the trim or of the elevators as happened to a Boeing 747, as documented here.
The causes of tail strikes are mainly:

Improperly Set Elevator Trim or Mis-Trimmed Stabilizer
Rotation at Incorrect Speed or excessive rotation rate
Improper Use of the Flight Director : The autopilot, of course, is not to be used for take off, but only when airborne after a certain altitude, if the F/D is displayed, also, the pilot should not follow its pitching order at rotation but only after; all Boeing and Airbus FCOM explain this.
Unstabilized Approach - flight crews who continue an unstabilized approach below 500' AGL will likely never get the approach stabilized. The result is a tendency toward large power and pitch corrections in the flare, often culminating in a substantial nose up pull at touchdown followed by a tail Strike.
Excessive Hold Off in the Flare and the pilot instinctively increasing pitch to try to prevent a hard landing. The resulting nose high attitude can lead to a tail strike. A good landing is one made at the right speed and close to the optimum touchdown point at a reasonable rate of descent.

At this website you will find every detail about tail strikes the causes, their effects, the training to avoid them etc.


text:
Are gliders susceptible to bird strikes?
Gliders don't travel at the high speeds that make bird strikes so devastating in airliners. However, they are also smaller, lighter and softer, and can still move pretty swiftly.
Are they also vulnerable to bird strikes? What happens when a large bird collides with a glider?

Gliders, as every other aircraft, are susceptible to bird strikes. This is a detached glider's tail after hitting a 10 kg vulture:

The  accident  was  caused  when  the  crew  lost  control  of  the  airplane  following  the  loss
  of a part of the tail assembly after the vertical stabilizer on the airplane struck a griffon
  vulture head-on.

Some countries are more susceptible to this kind of accidents than others. In Spain, which has a large population of vultures, I routinely hear about fatal accidents due to bird strikes.

More of a midair collision than a bird strike lol.

Great if you had a source for the fatalities per year.

"Injuries ... Crew ... Fatal: 2"

@FooBar I couldn't find any official statistic and have therefore removed the estimation, but you can find many other accidents such as this one or this one.

You aren't going fast enough for it to be a problem for fibreglass and gelcoat unless the bird is quite large, like a large eagle or vulture.
You encounter birds all the time in a glider.  You soar with them.  You spot one thermalling, you join him.
A hawk or eagle will pretty much ignore you unless you pass within 10 ft.  I encountered a large brown soaring bird, probably a juvenile bald eagle (they don't get white heads until they are several years old), last week at about 3000 ft in a thermal and passed under it several times, close enough to see it looking back and forth to keep track of me. Then he was gone...
To them a glider is just another big bird.
On tow, you have the tug in front making a racket that drives birds away, so that's not a problem unless the tug runs through a flock of gulls or something on departure.
Some eagles have been known to dive and attack gliders if you fly under them.  Bad news for the bird if it takes on the leading edge.  I've never heard of a composite glider being substantially damaged by a bird strike but I suppose it's happened and could be a problem if a large bird hit the horizontal tail or the wing while diving.
For me, bird encounters are always the highlight of any soaring flight.

Huge difference between "bird encounter" and "bird strike"!

I wonder if you'd like to revise the first sentence in light of the other answers; it seems conclusively proven that large birds do have enough kinetic energy to be a threat to composite gliders.

Yes made some edits thanks.  I soar where you rarely encounter really big birds, mostly hawks.

When juvenile hawks are competing for new territory in the spring, they'll stoop at radio-control gliders (typical wingspan 1.5 to 3 meters).  But you can usually outmaneuver them, and after a few such climbs and dives, they wander away in a huff.

I had a glider collision with what I believe was a golden eagle (possibly a juvenile bald eagle). It dove at me while I was thermalling. I think it was trying to scare me, but must have misjudged and slammed into my leading edge.
Did quite a bit of damage, but didn't affect flight characteristics and I was able to land safely. Unfortunately I watched the body of the eagle fall a couple thousand feet to the ground.


text:
Do high-wing aircraft represent more difficult engineering challenges than low-wing aircraft?
Generally, it's easier to make things strong in compression than in tension.
In a low-wing plane, the weight of the aircraft is on top of the wing; in a high-wing aircraft, it hangs from it.
It seems to me (I'm not an engineer) that the area of attachment in the latter case has to do a lot more difficult work (suspending the rest of the plane by its bolts) than in the former (bearing the weight from below).
And since in a high-wing aircraft all the structure is in tension (everything is hanging from something above it), presumably it's not just the wing and its attachment points that are affected, but most of the fuselage that has to withstand this tension.
Are these intuitions true, and if so, what are their engineering implications?

I dispute your premise - compression is only easier than tension when you're building in something like brick, stone or concrete, which is much stronger in compression than tension. Compression causes buckling of long members and needs careful design, while the strength of a wire in tension is almost unaffected by its length and is easy to design. Compare, for example, the tower of a tower crane with its lifting cable.

If you remove the first sentence it makes a lot better question. "Generally, it's easier to make things strong in compression than in tension" detracts from your question as it is not true in every instance.

The cynical answer is a well known aircraft designer's joke (though like all good jokes there is some truth to it). Civil aircraft have low wings because the passengers would get scared if they saw the cracks on the underside of the wing opening up when the plane took off :)

@ghellquist Well, that's why I wrote generally and didn't write always.

@DanieleProcida The not "always" case include sheet material that is stronger in tension than compression like paper (try compressing a sheet of paper and you will end up folding it). And sheet material includes things like sheet aluminium - you know, what airplanes are made of?

@slebetman Keep your sarcasm to yourself please.

@ slebetman Obviously paper is not designed to bear weight, but have you heard of cardboard?  Shaping paper into many little arches and gluing it makes it, well, more like wood!  Metal can also be bent and formed into shapes that COMBINE the tension/compression (the cantilever) properties into a very strong structural member.  But, yes, tension strength of paper (doped), Monokote, or sheet aluminum as a stressed skin greatly improves overall strength.

@DanieleProcida I wasn't being sarcastic. I was just reminding you that your non-always cases include airplanes

When practically everything that an aircraft is made of is easier to make strong in tension than in compression, it's misleading to say that it's "generally" the opposite; even if it were generally true when you count all structures, the materials that we use in structures that are not aircraft are irrelevant to the question. But I think the first sentence is an important part of the question; if you had not written it, how would you find out the premise is wrong?

Actually, in aircraft construction tension is preferable to compression: aeroplanes are thin walled structures, and compression forces introduce buckling.
In a low wing aircraft, the fuselage is pressing downwards on the top half of the wing, the bit that is under compression. In fact, quite complicated frame structure members are required for the fuselage/wing intersection for low wing aircraft: they need wing dihedral, so the wing looks pre-buckled at the spot of largest bending moment.

So although high vs low wing does have some differences in structural implementation, those are not the deciding factors in the layout. The design considerations for operational use are what drives the choice high-low-mid wing. Picture below from Torenbeek, depicting the Galaxy C-5.

High wing designs are usually applied for aircraft that need quick loading/unloading, and/or operate from airports with limited ground equipment.

Wing out of the way: good for loading/unloading, and for long extended flaps on STOL aircraft.
Floor close to the ground: easy cargo handling, good access for passengers, no need for airstrips.
More room for propellers.
Lowest induced drag at high lift.
Self stabilising roll behaviour.

Mid wing has the lowest drag of the three layouts at high speed, but poses a particular problem in running the wing through the fuselage. The wing must be a complete structure, because it has the highest bending loads in the wing centre and we don't want any holes there, best to make the hole in the fuselage to lead the wing through. But this bit of the fuselage cannot be used for passengers or cargo.
Low wing is usually applied in passenger aircraft because:

The undercarriage can be easily retracted.
The wing forms an impact structure that absorbs energy in case of a crash. Although the fuel tanks are in the wing...
The wing is fully underneath the floor and does not impede on the thoroughfare in the fuselage.
Optimal use of ground effect during take-off and landing.

In the 1950s, for the F27 the decision was taken to implement a high-wing design for its intended successor to the DC3: market analysis showed a significant market share outside Europe and the USA, at airports without the latest facilities. From wikipedia:

while a high-mounted wing had been selected as it produced a higher lift coefficient than a lower counterpart, it also enabled easier ground loading due to a lower floor level and provided unfettered external views to passengers without any weight increase.

And who wouldn't want unfettered external views?

Low wing for passenger jet aircraft is more specifically because the high speed of jet fans means they basically explode when they fail. Fast-moving shrapnel from shattered engine blades is often not contained within the engine housing, and would easily penetrate the fuselage. In contrast the wing is already strong, and  can take significant damage without affecting airworthiness. Putting the wing between the engines and the passengers is a major improvement for safety.

@Graham except a good number of airliners do not have the wing between the engines and the fuselage. Since the advent of high BPR turbofans, the engines tend to be ahead of the wing, with line of sight to the fuselage. That is the reason for the regulations concerning blade-off behavior of fans. I am pretty certain the reason for high vs. low wing has more to do with military transports requiring better ground clearance and FOD avoidance on unprepared fields.

@Graham yes safety is better with a low wing. There are many other considerations though.

@Graham: I challenge you to prove the statement that low wing is choosen to protect against shrapnel from shattered Engine blades.

@Graham your premise about containment is not backed up by the facts. The number of blade failures is low in any case, and most of them are contained and never become media stories. Another counter-example is propellers, which have no containment system at all when they fail.

The intuitions depend on the application.  Wood is very strong in compression, steel in tension.  And we must also consider G loading forces, which only add to the situation.
Airplane designers, over the years, have learned to use sound fundamental structural concepts to advance from opposing tension cables (very strong, not aerodynamic) to cantilever design (loaded triangles in both tension and compression), distribution of load (stressed skin), and tubular design (arch strength), as well as improved building materials
such as aluminum, steel alloys, and titanium.
Although attachment to a high wing as opposed to resting on a low wing does make sense,
the greatest loads are on the wings themselves, and the parts of the fuselage bearing the bending force of elevator and rudder.
So you have a very strong fuselage either resting on or suspended from the wing spars.
Military transports seem to favor high wings, airliners low wings.  No strong evidence for either case.  But a lot of bolts will make it strong.

Actually, wood is stronger in tension than in compression.

Beg to differ (I usually don't with your work). Trees are made by nature to support great weight (really tons) and also be flexible to the wind.  The concentric growth rings of a tree, combined with its composite matrix of cellulose polymers and natural resins gives it great strength to weight ratio.  The fibre has to be pulled apart sideways against the rings in order to fail.  No doubt also strong in tension as well, but designed for compressive strength.  Early aircraft designers complimented this trait with the tensile strength of steel cable. I would not mind building one from scratch.

@RobertDiGiovanni Have you considered that the "be flexible by wind" require wood to be very strong under tension - sometimes/often even stronger than under compression - to prevent wood from snapping in half? In my experience with model aircraft structure in a crash or structural failure (flutter, high G forces, etc) it is ALWAYS the inner part buckling that cause the failure rather than the outer part tearing - this proves to me that the wood I use (balsa) is much stronger in tension than compression. Foam on the other hand behave the opposite where it is ALWAYS the outer part tearing

OK, what KIND of wood.  The average stick broken over your knee fails tension first.  Even more so with "green stick" fracture. Balsa is a very soft wood and would certainly not be something I would use for structural strength with a full scale aircraft. The ancient Greeks knew how to complement steel cable and wood when they built their ships with a steel cable from stern to bow.  A wave at the ends would bend and pitch the ship, but one in the middle would break its keel without the cable. So the BEST of both are used together.

The issue with tension is that when any one part fails the whole structure fails.  Quality control of natural wood is much less consistent than metal.  Also cables and ropes are braided together with multiple strands to help reduce this risk.  But no, sorry, can't go with wood over steel cable in tension, but I would surely consider plywood skin over metal to improve strength.

This document suggests that wood is is stronger in tension than in compression. (Page 4-24 has a table of tensile strength) Often by a factor of 3 or 4.

Snapping a stick (whether or not you force buckling with your knee) isn't a good measure of comparing tensile and compressive strength, because it depends one how ductile and malleable the material is as well.

These are very good points.  Wood provides stiffness, metal tensile strength in practice.  And we certainly cannot generalize "wood" as one material, there is a huge range.  As far as "tensile" as pulling apart compared with twisting or bending, particularly in fatigue or cracking, yes, we can work on more definitions or tests.  Wood remains an amazing natural composite material, the Mosquito was mostly wooden, also the "Spruce Goose".  With mass production, you run out of trees.

@RobertDiGiovanni Live trees have great bending resistance due to the tension of the fluids inside, like a balloon filled with water buckles less readily than an empty one. Dead trees are blown over by a storm. Bending resistance is what is required to resist storms, and is not the same as compression strength of the dead material.

the tensile-versus-compressive stress issues have been worked out to a satisfactory degree many years ago, meaning that the loadpaths for high-versus-low wing aircraft really aren't design differentiators- but there are other issues, as follows.
Low wings furnish a natural location for a wide-stance main landing gear, making for stable landings and easy ground handling. But high wings are less prone to damage from striking rocks or bushes on the ground.
In a low wing layout you can position the pilot and copilot seats over the main wing spar so they do not reduce cabin room, whereas a main spar carry-through in a high wing layout might reduce headroom in the cabin. However, a low wing interferes with the pilot's view of the ground whereas a high wing does not.
These differences- which do not have anything directly to do with stresses in the airframe- affect the pilot's decision-making process with respect to buying and flying a low wing instead of a high wing plane.
I invite the experts here to add their comments.

Flying a turn will make the high wing obstruct the pilot's view; only ground view favours the high wing.

For structural weight efficiency, tension wins because stiffness isn't a factor.  This means, if structural efficiency is your top priority, a high wing, braced with struts, or for even less weight cables, wins.
With strut bracing, the major structural attachments are simple pin joints, and the highest stress component, the wing strut, is in tension except during reverse or negative loading where it's in compression, but where the requirement is less.  There is moderate compression loading along the spar axis directed to the wing root, and along the upper spar cap at the strut attachment, but nothing like the compression stress in a fully cantilevered structure at the wing root.
And for best visualization, really take it to the extreme.  Look at a paraglider.  You can't compress a string.  The wing is "high" and everthing is under tension load.  And the whole thing weighs maybe 10lbs but can lift 200+, or 20+ times its weight.
Note that on cantilever high wing airplanes, like a military transport or a Dash 8, the placement of the wing has little structural advantage and there are other issues to favour one or the other, like loading etc.

Cessna Cardinal is high wing with cantilever beam, no struts. Fixed gear and retractable. Pilot sits slightly ahead of the wing for a great view, and there are no struts to block the side view. Very nice plane to fly.  Cessna 210 and P210 are also high wing strutless.  I know the pins that hold the ends of the wing spar to the fuselage are pretty big.
http://www.crossroadsfencing.com/airplane/painting%20pics/IMG_0563.JPG


text:
What is an "all-weather" aircraft?
I've read a few times that such-and-such a plane is an "all-weather" aircraft, for example:

The McDonnell Douglas F/A-18 Hornet is a twin-engine, supersonic, all-weather, carrier-capable, multirole combat jet.

This is quite intriguing and a rather odd thing to specify.
A fighter that couldn't be operated in for example rain (perhaps because it might dissolve, rust or leak) wouldn't seem to be much of an asset to one's air force.
Similarly, I would not expect a supersonic fighter to be unable to deal with gusty winds, or a heatwave, or really, anything else the weather might be expected to throw at it.
What sort of jet would not be an "all-weather" aircraft?

Do weather conditions which cause icing account? If so „all weather“ is a little bit thin in meaning, jet fighters have no anti-icing for the flight surfaces. https://aviation.stackexchange.com/questions/58982/do-fighter-jets-have-deicing-in-their-wings

“All-weather” is a little bit of a hold-over from the WWII days where severe weather was much more of a threat to the aircraft. Between less reliable engines, delicate and finicky instruments like artificial horizons, and limited anti-ice technology, flying in adverse conditions was much harder then. Becoming completely disoriented in low visibility was almost a death sentence.
Special planes were made with the sole purpose of fighting at night, and certain designs were better than others for all-weather operation.
Nowadays, world class military aircraft are all pretty much all-weather because of the 70 years of avionics, propulsion and airframe improvement.

Up to WW2, aircraft operations were highly dependent on weather.

navigation used ground observations to correct for drift etc.
military aircraft needed optical observations to find their targets (on ground or in the air).

This limited aircraft use to daytime and put an upper limit to the amount of clouds that were acceptable. A target completely covered in low-level cloud meant you couldn't find it.
The same goes for the landing. You had to find the airfield visually. Descending blindly through clouds could end up with you crashing into the nearby hill.
Then in WW2, radar and radio navigation aids were used for the first time. This allowed some aircraft (the ones with proper equipment) to function day and night. The equipment was too large and expensive to be installed in every aircraft, so you got a bifurcation: a large amount of day fighters (without radar), and some night fighters (with radar). Similarly, bombers were flown in groups where only some of them had radio navigation (Pathfinders) and the rest followed the leader. Later on, the equipment got cheaper and smaller, and from the 1960s night fighters ceased to exist as a separate class: pretty much all new fighters were all-weather capable.
So an all-weather aircraft needs radar, accurate navigation, and an ILS.

All-weather fighter aircraft means it can operate in low-visibility such as night time.
For example F-16 was envisioned as a cheap day-fighter, but ended up being an all-weather fighter.
See: https://en.wikipedia.org/wiki/Night_fighter

In the context of a fighter this can mean not only that the aircraft can operate safely in (almost) all weather conditions, but that it can be effective in combat in weather also. This means airborne intercept radar and homing missiles, vs older fighters that relied more on the pilot's vision and guns.


text:
How much does (or can) the fuselage of a long aircraft flex?
How much will the unsupported ends of an airliner's fuselage droop in level flight?
In turbulence, how much vertical movement will they experience relative to the wing?
And finally, in extreme conditions, what is the expected limit of such movement?
Is it a movement of a few millimetres or a few centimetres, or more than that?
I assume any horizontal flex - or torsion along the axis - will be much smaller.


text:
What does the behaviour of water on the skin of an aircraft in flight tell us?
Sometimes in damp conditions I notice some surprising (surprising to me, anyway) behaviour of water.
I'll see moisture glistening in the heads of rivets or bolts on the top of the engine pylons - that stays there for a long time (many minutes) after take-off.
Similarly, at the front edge of my window I will see a pool of water that only very slowly diminishes, seeding little drops (that become smaller and smaller as airspeed increases). Again, this pool remains far longer than I'd expect.
I'm surprised because I'd expect the evaporative and sweeping effect of the high-speed airstream to clear this moisture extremely fast, and yet it doesn't.
I suppose that despite the speed of the airstream, there are numerous small areas on the surface of the plane that are sheltered from it very effectively.
I'm interested to know what the implications of the causes of this effect are are for aerodynamics, for example how they relate to drag on the fuselage, or - where it happens on lifting or control surfaces - how their operation is affected.

Consider observing your wing's leading edge while in flight and see what happens there.

Sometimes the water is solid, which hopefully tells us something.

What evaporative effect? Air at 100% humidity does not evaporate anything at all.

The rivets and bolts are (thermally) connected to larger pieces of metal.

This effect is likely caused by something called the "boundary layer," first defined by German engineer Ludwig Prandtl in a paper he presented in 1904 at the third International Congress of Mathematicians in Heidelberg, Germany. From Wikipedia:

The effect of the paper was so great that Prandtl became director of the Institute for Technical Physics at the University of Göttingen later in the year.
Over the next decades he developed it into a powerhouse of aerodynamics, leading the world until the end of World War II. In 1925 the university spun off his  research arm to create the Kaiser Wilhelm Institute for Flow Research (now the Max Planck Institute for Dynamics and Self-Organization).

The math in the paper is complicated and frankly well beyond my paygrade, but the way I understand it, what I think you are seeing is caused by a laminar boundary layer that forms very close to almost all surfaces of the aircraft. It is a layer of air with high viscosity, low turbulence, low speed relative to the air farther out from the surface, and causing an increase in drag. That's why the water doesn't blow away or evaporate.
The image below is one I found which does a good job of presenting the two types of boundary layer -- laminar and turbulent. Look at how smooth the laminar boundary layer looks compared to the turbulent boundary layer. This might help to understand the concept better.

The boundary layer is also something I experienced with an experimental aircraft I restored that had an opening on the top right of the fuselage to feed air into the fuel injection system intake. As it was, air would essentially stop flowing into the system when the aircraft accelerated, and the engine would stop. Not good. In order to stop that from happening, a raised scoop had to be installed that extended above the laminar boundary layer and into the turbulent boundary layer. That solved the problem.

The issue that you mention is why many aircraft air intake systems are separated from the fuselage. This is probably most clearly represented on the F16.

Depending on where one is looking on the aircraft, the boundary layer is probably turbulent and not laminar. That said, it is still comparatively low speed even when turbulent, and there is a thin region on the order of a millimeter that is "laminar," even for turbulent BLs. A BL is also why insects walking on your car hood or windshield tend to stay around much longer than expected while you're driving around. It isn't confined to vehicles either -- the reason wind is only a few MPH at the surface while it's much, much faster at higher altitudes is due to the atmospheric boundary layer.

Yes, a laminar boundary layer will eventually become turbulent the farther back along the surface of the aircraft you look. I don't know the theory well enough, but clearly devices like wing fences and vortex generators are intended to break up that laminar boundary layer much earlier along the chord of the wing, for example. The explanation for this is for another question. :)

The moisture stays at the surface because the air speed at the surface is essentially zero. (No-slip condition)
A solid body moving through a fluid develops a boundary layer around it, which is caused by friction between the surface and the air.
A boundary layer is a region of slow moving air relative to the body it surrounds. The closer you get to the surface, air velocity relative to the body decreases. This is why dust doesn't really blow off your car even when driving at highway speeds.

Source: http://img.tfd.com/mgh/cep/Typical-laminar-boundarylayer-velocity-profile.jpg
As to why the pool of water slowly diminishes - As the airspeed increases, the boundary layer
becomes more turbulent which has higher shear stresses and higher rate of mixing within the layer.
The drops of water around the aircraft do change the shape of the surface locally, same with insects or icing which degrades performance (increased drag, forced transition to turbulence), but the effect is very small.
What does this behaviour of water on the aircraft skin tell us?
Not much really. It's hard to see clear water.
Aerodynamicists use oil with dyes instead to visualize the airflow around the surface of the aircraft.

Source: https://www.pinterest.ca/pin/122863896054044619/
Using this technique, they can spot regions of flow separation and reattachment, spanwise flow and more. This information helps them refine the aerodynamics of the plane.
Edit: fixed grammar + added pic

The profile is only good for a laminar boundary layer. Most aircraft boundary layers are turbulent as are basically all of the car boundary layers.

Not closest to the surface. That boundary layer is laminar, with varying thickness, which then turns turbulent.

I'm not an aerodynamic expert by any measure so take this with a grain of salt, but from what I understand, this is because high speed air around a solid body forms a boundary layer. Below the boundary layer, the air immediately around the solid body is moving together at a very low speed relative to the solid body and this reduces to zero on the surface of the solid body.
The reason why the water is able to hold on for long is because, even though the plane is moving really fast, and that the air speed is really high outside the boundary layer; inside the boundary layer, the air is relatively still.

Welcome to Av.SE!

The main thing it tells you, in practical terms, applies when it's below freezing. That snow you think is going to blow off when you take off will not blow off wherever there is a significantly thick boundary layer (it will generally blow off down to the depth of the boundary layer, which can also be present at the leading edge and although very thin, it can leave enough surface roughness to kill you), which is why airliners are deiced everywhere, not just at the key areas of the leading edges.

authors: Vladimir F Героям слава


text:
Did airlines fly their aircraft slower in response to oil prices in the 1970s?
Modern airliners generally have lower cruise speeds than those of comparable aircraft from 40 years ago (the venerable Boeing 737 seems to be something of an exception; it has always had a more leisurely cruise speed).
Airlines had to respond much faster to the oil crisis than manufacturers could - to what extent were airlines able to economise on fuel consumption by flying more slowly, and what sort of margin was available to them at cruise (and by extension, what sort of margin is available in modern aircraft, should economic factors drive operators to seek lower fuel consumption by flying more slowly)?

related: What is Cost Index? (in short, "the ratio between the unit cost of time and the unit cost of fuel")

As a French who grew up in the Caribbean, I can tell that flights are getting longer. A flight between France and my island that took 7.5 hours 15 years ago is almost 9 hours long today.

This post published by the MIT’s School of Engineering says (emphasis mine):

Specified cruising speeds for commercial airliners today range between about 480 and 510 knots, compared to 525 knots for the Boeing 707, a mainstay of 1960s jet travel. Why? “The main issue is fuel economy,” says Aeronautics and Astronautics professor Mark Drela. “Going faster eats more fuel per passenger-mile. This is especially true with the newer ‘high-bypass’ jet engines with their large-diameter front fans.”

Today's high bypass engines are more efficient at low speed:

Observant fliers can easily spot these engines, with air intakes nearly 10 feet across, especially on newer long-range two-engine jetliners. Older engines had intakes that were less than half as wide and moved less air at higher speeds; high-bypass engines achieve the same thrust with more air at lower speed by routing most of the air (up to 93 percent in the newest designs) around the engine’s turbine instead of through it. “Their efficiency peaks are at lower speeds, which causes airplane builders to favor a somewhat slower aircraft”

This article also states that slower aircraft's structures are cheaper to build:

“A slower airplane can also have less wing sweep, which makes it smaller, lighter and hence less expensive.” The 707’s wing sweep was 35 degrees, while the current 777’s is 31.6 degrees.

Interesting information, and thanks, but I don't think it actually addresses the question: did airlines start to fly their planes more slowly? (which is not the same as did airline manufacturers start to design slower planes?).

If modern engines are more efficient at lower speeds, I imagine older engines were so as well. Just about any engine is more efficient in a region below it's maximum capacity.

@DanieleProcida airlines bought slower aircrafts overtime :)

I find this a bit dubious, as airliner cruise 'speeds' are rarely specified in knots. My understanding is that it's usually rather a specified cruise Mach number, which represents a different airspeed depending on altitude and atmospheric conditions.

@Mast Older engines were low bypass which makes them more efficient at higher speed

@Mast The 707 used low-bypass turbofans (bypass ratio around 1.4:1,) not high-bypass turbofans.

Right. Besides that, any aircraft is likely to be most efficient in the range it's supposed to work in (cruising speed), so going above or under it is likely to be inefficient anyway.

@QuentinHayot Yes, they did buy slower aircraft eventually - but can you add to your answer to say what they did at the time with the aircraft that they already had?

The 7.5 to 9 hour bit at the beginning of the answer also surprises me. I'm not surprised that it's a bit slower (the 747 was the main option 15 years ago and it's one of the fastest airliners out there, so replacing it will slow things down a bit,) but by that much seems surprising. I would guess that at least part of it is explained by other factors, such as difference in time of year (and thus wind), more congestion at the airports, routing, and schedule padding. Of course East-bound vs. West-bound will also be significantly different due to prevailing winds.

@Mast The 707 would have been most efficient at its (higher) cruise speed, but would still have been much less efficient than newer airframes with high-bypass engines at their most efficient (lower) cruise speeds.

@reirab I did the round trip at the same time of the year every year all my life. The 747 was definitely quicker. I remember it was consistently 7.5 hours in one way and 8 hours on the way back. Today we fly 777's or A330/A350 and its around 8.2 and 8.5 up to almost 9 sometimes. And they can definitely go quicker because it happens that they will drop to 8 if they are very delayed and need to make up for it.

@QuentinHayot Yes, the 747s are definitely faster, just not enough to go from 7.5 hours to 9 for that reason alone. The 747 (-400 and -8) cruises at Mach 0.855. The 777 is Mach 0.84 and the A330 Mach 0.82. And you are correct that this is substantially less than maximum Mach number in each of these cases, so they can fly a bit faster if they need to at the expense of a bit more fuel... assuming that traffic allows.

@reirab you are probably right and my memories may compare best/worst performance. But there is definitely at least 1 hour of gap regardless of the Mach value. Maybe they don't fly exactly the same routes or whatever.

15 years ago was also just when the EU air passenger rights regulation was introduced. As its implications become clearer (and especially after the ECJ ruled that the cash compensation the regulation sets out for cancellation should also be paid in case of delays), airlines would have a definite incentive to construct their long-haul schedules more conservatively. Nobody complains when their flight arrives early.

Contingency, and not so much from the 70's but the influence of Jan Carlzon in the 90's. Getting the cost below 10c per pax per Km factors in, yes. But it is not free to use airports and schedules matter. Building in a contingency helps pilots make up time. I would like to think they burn less fuel carrying fuel as well, but they also need that fuel for contingency.

It would be interesting to see the math on fuel savings per flight vs fielding more flights with the same fleet. Obviously personnel, maintenance / inspection times, etc make things a lot more complicated than just fuel efficiency alone.

I have only my own anecdotal evidence. I was flying domestically & internationally quite extensively during the 1970s, and particularly the period spanning the oil crisis.
Although the impact of the 747 on mass air travel had begun in some markets, most aircraft were still legacy assets, often belonging to government owned national carriers that were still run by pilots, engineers & bureaucrats, not accountants & MBAs. It was very common on long international flights to have aircraft that were only half-full, if that; my first flights between Australia & the US or Europe were spent stretched out over three seats, this was considered to be commonplace at the time.
The airlines reacted to the oil crisis by immediately cutting the number of flights, & getting rid of a lot of bulk-buy cheap ticket deals to travel agents. In many cases it was a physical shortage of fuel as well as the increased cost that excused what led to sudden huge increases in queues at airports that were still just glorified empty hangars that had never dealt with crowds.
Long distance aircraft were constrained much more by having to stop & refuel much more often than they do now; what is today a single hop from Sydney to DFW, was then Sydney to Fiji, then Hawaii, then LA, then domestic.
I don't recall the aircraft going much slower, or at a different altitude (not sure if that was practical), but there may have been some adjustments with somewhat less flexible legacy aircraft such as the 707s & DC8s. They were already flying as efficiently as they could, just to avoid having to land. With fewer aircraft flying, they were now more likely to be operating closer to their maximum weight, which constrained performance.
Early 747s were more economical to start with, but when they were first introduced they had far fewer seats than post-oil crisis, when they were quickly reconfigured to get very crowded. Pre-crisis you preferred a new 747 for its space; post-crisis you hoped for a DC8 because they couldn't rob you of seat width.
With domestic air travel, again, they just cut flights & charged like wounded bulls. There were many non-flying areas in their operations that were hugely inefficient that could be made less costly, & these were all overhauled. Political lobbying became much more important in order to reduce technical & employment regulatory "costs". Refuelling at lower-cost airports & tankering changed scheduling.
TLDR, yes, they probably slowed aircraft slightly, but that wasn't nearly enough - instead the airlines cut flights massively & ordered smaller seats for their 747s. It took years for airlines to recover, many just went bankrupt.

They were already flying as efficiently as they could, just to avoid having to land. Efficiently in terms of what?

@bautista - In terms of fuel consumption per mile flown.  The idea is that if you fly trans-oceanic excessively fast, you will have to detour to land to refuel more often, and therefore truly long distance passengers will get to their destinations later while paying more.

That's some anecdotal evidence you got there. I feel like an expert on the impact of the oil crisis on airlines just from reading that. Thank you.


text:
How have fuel consumption and emissions per passenger/kilometre declined since the first jet airliners?
The efficiency of jet aircraft has improved since the 1960s.
Is it possible to chart figures that show how consumption and emissions of pollutants and CO2, per passenger/kilometre, have declined since then?

Don't know where you can find those details (ICAO or some UN organization maybe?) but you could ballpark it by looking at the specific fuel consumption of a 1950s airliner engine and a modern one.
The JT3/J57 turbojet used on the 707 in the late 50s had a specific fuel consumption of .909 lb/(h lbf) at cruise, and the GE90 used on the 777 is .545.  This means the fuel burn per unit of thrust is about 40% lower on the GE 90 than the JT3. Since a jet engine is just a fancy oil furnace, without elaborate systems to reduce specific kinds of emissions like car engines, the SFC itself should be a good  proxy for overall emissions.
Being conservative by assuming the thrust per pax is roughly the same for modern aircraft vs old ones (I'm sure it's lower), you could use that comparison as representative of the industry and say emissions are at least 40% lower per seat mile now than in the 50s.


text:
How are runway lights constructed?
How are runway lights constructed so that they are not damaged by aircraft wheels, and can continue to function safely, to demanding specifications, under a wide variety of environmental conditions?

You’re talking about flush runway lighting?

@CarloFelicione Any runway lighting that is at special risk of being damaged by aircraft wheels, shedding broken parts onto the runway, ceasing to function adequately, etc.

I presume you are asking about lights that are on the surface of the runway like centerline lights and touch down zone lights. The FAA has an AC that covers their design and installation which you can find here. Take a look at section 4.5 "In Pavement Light Fixture Testing". This section outlines some of the force requirements the light must be able to withstand. The FAA currently requires the light to be mounted in an L-868 base can which is imbedded in the runway during construction. There are also a bunch of size requirements laid out in section "3.4. Dimensional Requirements." that describe how the lights are to be built.

A runway edge light won't survive an encounter with an aircraft wheel.  They're just heavy duty light fixtures on short posts.  Heavy glass lenses, well sealed, with materials that can survive outside exposure and stand up to snow shot across by plows, etc.
If an airplane takes out a runway light, there will be much more excitement over the follow-on activities happening to the airplane compared than with runway light's fate.

There is no protection.
While dead heading in the copilot seat, the pilot of a Beech 18 cut a corner too tight and mowed a couple down (Kodiak Alaska). An animated discussion with the airport manager soon followed. Close inspection revealed the light posts slip into a base and are held with set screws, it broke and bent parts off the post and base.

source

Just to nitpick, that's an image of a taxiway edge light, not a runway light


text:
What was the first aircraft designed with a plug-system fuselage, and why?
Nearly all modern transport aircraft are designed with a plug-system fuselage, allowing longer and shorter variants to be built by adding or removing plugs.
In addition, the plug system simplifies construction, as large sections of the aircraft can be assembled from modular parts.
What was the first aircraft to employ this system?
And what was the reason for adopting it in the first place:

to make possible the construction of variants
to enjoy efficiencies and economies of design and manufacture
both of the above?

@ymb1That does not say anything about why the designers of the first airliner to adopt the plug system did so.

Side note: This comes from marine ships, which have used this even before airships.

This should be a Zeppelin. You said "aircraft", not "airplane", so lighter-than-air systems qualify also.
Already the model submitted with the patent application in 1895 (yes, back then the inventor had to demonstrate that the invention is actually implementable!) showed several cylindrical sections, akin to a train of cylindrical balloons. The final patent (Reichspatent No. 98580) was for a "Lenkbarer Luftzug" (dirigible aerial train). Adding more sections would increase length and payload.
Later, when the maximum diameter of Zeppelin airships was limited by the size of available hangars, their mid section was of constant size and allowed to add plugs to increase payload and range. This was, among others, the case with LZ 104 which turned out 30 m longer than initially planned.
The last big airliner not designed for stretching was the Lockheed Constellation. It had a continuously varying cross section from tip to tail.

Interesting, though if it is the balloon that is made of sections, does that count as a fuselage?

@DanieleProcida: Good question! At least it was the part holding the payload, and the idea was to spread the payload over the length of the airship. Only it is not called a fuselage.

In the US, that was most likely the Douglas DC- series of aircraft. Douglas used the plug-stretch design principle throughout the series, as more powerful engines became available over time which permitted more payload.


text:
Does the nearest function on a G1000 allow for listing of anything other than nearest airports?
In non G1000 GPS systems, I have seen several pages for selecting the nearest airport, VOR, NDB, intersections, and waypoint, but in a G1000, I can only seem to get a listing of the nearest airport, but have to manually enter VORs.
Am I missing something, or does the G1000 not have nearest functionality for these other categories of identifiers?

The G1000 and G500 do indeed have pages for the nearest VOR, NDB, Intersections, and user defined waypoints. The  G1000 Integrated Flight Deck Pilot’s Guide gives a list and flowchart / interface map for these.

Turns out it only works on the FMS knob for the MFD, isn't that right?

If you mean that the large view “Nearest” page only works on the MFD, you are right. This large view has several “Nearest” options. The small, bump out window on the PFD should show a list of the nearest airports. But, NDBs, VORs, etc, don’t seem to be available on the PFD.

"NDBs, VORs, etc, don’t seem to be available on the PFD" - This is what threw me off and caused me to ask this.


text:
How often do large airports clean up the black stuff?
Looking at Seattle International as an example, the first third of the runway is covered with a dark streak of tire material that accumulated from landings.

How often, if ever, is this cleaned up?  How is it done, and does it leave a lot behind after the cleaning process?
Does too much of this black material create a safety hazard in any way?

Related: How is tire rubber removed from the runway?

Voted to close as a duplicate.


text:
How does the FAA define "Endorsement"?
I've looked in Part 61.1 and Part 1.1 (definitions), and looked for it in Advisory Circular 61-65H, "Certification: Pilots and Flight and
Ground Instructors", and a definition of "Endorsement" is not included among them.
What is the FAA's official way of defining what constitutes and endorsement?

Not sure if this is an exhaustive list, but AC 61-65H has a list of endorsements.

If it isn't defined in the FARs, then a common definition would be appropriate.

Page 1 of  Advisary Circular AC61-65H says:

Endorsements represent training requirements completed and privileges
granted. This AC helps airmen and instructors ensure that all training
is completed and documented clearly and concisely. An endorsement
marks and formalizes events such as an operating privilege or
authorization granted or a limitation incurred.


text:
Under what engine conditions is glide performance typically determined in performance charts?
The performance section of an information manual I am using states that the glide range indicated is "power off".
Does this terminology definitely imply that the engine is not firing? or could this also refer to an idle engine that is running, but producing nearly zero thrust, or even creating drag?
I assume, it means zero ignition, but cannot tell for sure from the document.

Power off means what it means.  No ignition or no fuel, and a windmilling prop which causes drag.

What sort of airplane is it?

This is regarding single-engine airplanes with reciprocating engines.

In Australia, it's with engine inoperative, see page 47, Glide Performance.


text:
Is it generally acceptable to use 100LL and 100 grades of AVgas mixed into the same tank?
Is it acceptable to use 100LL and 100 grades of AVgas mixed into the same tank, or should one type of fuel be drained prior to addition of a mismatched type?
This is assuming the aircraft is approved for the use of either of those grades.

I don’t believe 100 grade fuel is available anywhere. Only 100LL.

@MikeSowsun fair enough.  I've never come across it either.  I've had someone tell me that if you mix them, that the colors neutralize and the fuel becomes clear.  I don't know if that's true or an urban legend.

We used to mix green 100/130 and blue 100LL back in the 1970s and the result was definitely not clear. Colour charts say mixing equal parts green and blue yields cyan.

@MikeSowsun Shell claims it’s still available at wholesale, but I’ve never seen it anywhere at retail.

@Mike Sowsun: That's only if the coloring agents remain unaffected by the mixing.  If there's a chemical reaction between them, the result could be colorless.

You can mix them freely.
While the modern meaning and measurement is far more complex, octane rating originated as the percentage of actual octane (C8H18) in a sample of gasoline. This means if you mix equal parts 100 octane and 80 octane, the result is 90 octane.
The addition of lead makes gasoline behave like it has more octane than it actually does, which is how you can get octane ratings over 100. (Ethanol does this too.) However, that doesn’t change the simple math above. If you mix two gasolines that are both rated 100, even if they got there by the addition of different amounts of lead, the result is still 100.
Also, the amount of lead in the standards for both 100/130 and 100LL is a maximum, not a fixed value. Actual retail samples today have significantly lower lead content due to starting with higher octane unleaded, so far less lead is needed and in practice 100/130 is now likely to be 100LL just with a different color dye. It’s simply not worth doing separate production runs for each due to the enormous fixed costs to clean up the lead contamination of the refinery, tanks, trucks, etc.  It’s the same reason you can’t find 80/87 or 91/96 avgas anymore: there’s no point when they can just sell 100LL to everyone at a lower total cost.

Actually it is not quite a simple algebraic relationship. In the example given it might be 90.1, not 90 , but of no practical difference.

@blacksmith37 Do you know the correct formula?

Each component can have a slightly different response to lead. And each gasoline contains several compounds in different ratios , so pretty complicated. But auto fuel today ( in US) does not contain lead so there is no need .  I guess there is not much incentive to do it for avgas.  Each refinery blend would be tested for octane  with lead. A refinery makes different blends ; high elevation and high temperature need lower octane ( with natural aspiration).

The obvious answer would be: Check your manual. Technically, there shouldn't be any problems. However, if your engine is designed for 100LL and you use the mix over a long period of time, you might want to have your sparkplugs checked.
Apart from that, the environment is definitely not going to thank you for using leaded fuel...


text:
Could a pilot be in violation of an FAA rule for the use of an EFB solely as an aid to situational awareness?
Are there potential situations in which use of an EFB even as a supplement for situational awareness only would be considered a violation of a rule?
I understand that some operators do not have approval to use EFBs, and they are required to utilize paper charts.  Does that normally preclude the use of an EFB for supplementation as long as all requirements are otherwise met by onboard paper products?
I suppose one could say this is unlikely to happen for the sole reason that it's hard to prove what is being used onboard in-flight, and my question is more to the point of whether or not the use is actually allowed as long as it's only a supplementation.


text:
Why different colors (blue and white) for different airways?
Why some airways, (example T358) are drawn in blue, while
other (example V44) are drawn in white?
I can not see any difference between blue and white airways,
step between waypoints, direction and altitude restriction looks similar.

Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.

It would be worth mentioning in the question what software this is chart from, both to get good answers and for general interest. I'm aware of flightradar24, but (on my screen) that looks different to this


text:
What is the Eights on Pylons maneuver intended to demonstrate about a pilot's skillset?
This maneuver has the common requirements such as clearing the area, and it has the unique requirement among maneuvers to determine the pivotal altitude and select appropriately spaced pylons as references for the maneuver.  However, unlike others, this maneuver has no altitude, heading, bank (other than not to exceed 40 degrees), or airspeed tolerances to determine satisfactory performance standards.
Without these tolerances, what is actually intended to be proven of the pilot's ability upon successful performance of this maneuver?

This is the ACS page for commercial airplane eights on pylons:

The maneuver tests the ability of the pilot to hold precise altitude and airspeed. You need to make constant small corrections when flying into and away from the wind. In my opinion, it is the most difficult of the Commercial maneuvers.
Garry Wing has a good explanation of the technique.
As Garry Wing explained in the video. If you don’t hold precise altitude and groundspeed the point moves. So as the wind increases upwind it lowers your groundspeed requiring a lower pivotal altitude. Downwind you will increase your groundspeed requiring a higher pivotal altitude. So you need to remain coordinated, constantly vary the altitude, scan for traffic, and maintain situational awareness of your location around the pylons.
Your airspeed will vary a bit as you climb and descend and head into and away from the wind, but if you can’t maintain a consistent groundspeed your pivotal altitude will change and that will throw off the whole maneuver. As you can see in the chart below, if your groundspeed varies by 4 kts from 87 to 92 kts the pivotal altitude changes by 65'. This is the part of the maneuver that requires precise control of airspeed and altitude with coordinated use of the rudder and ailerons.

The FAA explains it well in Airplane Flying Handbook:

Eights-on-Pylons
The eights-on-pylons is the most advanced and
difficult of the ground reference maneuvers. Because of the techniques
involved, the eights-on-pylons are unmatched for developing intuitive
control of the airplane. Similar to eights around pylons except
altitude is varied to maintain a specific visual reference to the
pivot points.
Chapter Summary
At the completion of ground reference maneuvers, the
pilot should not only be able to command the airplane to specific
pitch, roll, and yaw attitudes but, while correcting for the effects
of wind drift, also control the airplane’s orientation in relation to
ground-based references. It should be reinforced that safety is
paramount in all aspects of flying. Ground reference maneuvers require
planning and high levels of vigilance to ensure that the practice and
performance of these maneuvers are executed where the safety to groups
of people, livestock, communities, and the pilot is not compromised.
To master ground reference maneuvers, a pilot must develop
coordination, timing, and division of attention to accurately maneuver
the airplane in reference to flight attitudes and specific ground
references. With these enhanced skills, the pilot significantly
strengthens their competency in everyday flight maneuvers, such as
straight-and-level, turns, climbs, and descents.

Can you please elaborate on how it tests the ability of the pilot to hold precise altitude and airspeed?

@RyanMortensen I edited my answer.

I was always under the impression that the pitch and altitude changed dependent upon the resulting ground speed changes and the only thing that was constant was the angle of bank and power.  In other words, altitude and ground speed are not intended to be constant, only bank and power.  Is that not so?  Actually, the last sentence of the quote block for E.O.P is "Similar to eights around pylons except altitude is varied..." and altitude varies proportional to ground speed per the table.

@RyanMortensen FWIW, that is how I was taught to do Eight-on-Pylons and how I taught my students when I was an instructor.

@RyanMortensen I believe that power is set at entry and not changed. Groundspeed and altitude are adjusted with pitch. Bank angle is dependent on where in the maneuver you are reltive to the wind. Just like any ground reference maneuver that keeps a fixed distance from a point or line, the steepest bank occurs when flying with the wind and the shallowest bank when flying into the wind.

@JScarry you're right. The Airplane Flying Handbook sure doesn't put a firm ban on alterations in bank, but it also indicates that changes in altitude can be used to correct the position of reference point on the air frame.  So, now I am wondering how you're supposed to decide when to do which as the corrective action.  Maybe wind drift errors = bank and turn radius errors = altitude?

The more I read into this subject, I am starting to get the impression that bank and altitude changes, when there is a wind, are actually interconnected since the rule appears to be the lateral axis must point at the reference implying that the reference point is the ultimate determiner of bank at any given point, but a constant distance from the pylon is absolutely not a maneuver requirement.


text:
What does “in position time” normally mean in the aviation industry?
I’ve read route descriptions for a cargo company, and they have a line for “in position time”.
What does in position time correspond to as industry jargon? Does it mean holding short of the runway, ready to taxi, at the flight planning room, or something else?

What can you glean from the other times surrounding it?  i.e. are there lines for show, engine start, takeoff, etc?  And if so, where in the order does this one fit?  It seems like you might be able to narrow it down based on context.

@Michael Hall I suspect that it’s the time to begin the takeoff roll, but it can’t hurt to ask. Once I find out 100% I’ll just answer my own question if it doesn’t already receive a definitive answer.

But what do the other lines say?  Where does this fall in the timeline of events?  Please tell us what you do know to help us help you...  (for example, if it occurs sequentially after a line for taxiing then it obviously can't be for the crew to meet "at the flight planning room", right?!  Surely there are other lines besides this one?)


text:
How many FAA-Certified Flight Instructors Hold a Gold Seal?
About how many, and approximately what percent of the instructors who are certified under the FAA hold a gold seal on their CFI certificate?
How many not including expired certificates capable of being renewed via a reinstatement ride?
How many including expired certificates capable of being renewed via a reinstatement ride?

As far as I can tell, this information is not publicly available. You may be able to submit an open records request (FOIA) to the FAA for this, but nothing on the FAA website provides information about gold seal instructors or certificate status.


text:
Why is the alternate static pressure lower than the standard static vent pressure regardless of the venturi effect on both?
I understand that the higher relative airspeed around the body of an aircraft in comparison to the air within the cabin causes the cabin to experience a lower pressure.
However, I was just thinking about this concept, and it dawned on me that I have never questioned why this causes a difference in this particular discussion as it relates to standard vs. alternate static ports.
Are the standard static ports on the side of the hull not also experiencing this effect as air passes nearly perpendicular to the standard static port openings as it flows along the body of the aircraft?
Why is the venturi affect stronger around the cabin than it is around the static port holes on the side of the hull at the same indicated airspeed?

Venturi effect stronger around the cabin? Venturi effect only happens if the space where airflow needs to pass becomes smaller. Around the airframe (and over the static ports) you have the boundary layer. ??

A venturi effect pertains to a change in pressure caused by a CHANGE in airflow velocity.  The static air pressure at the static port is not automatically lower just because the air is in motion there.
After all, the air within the fuselage (and within the piping leading to the static port) is in motion relative to the outside airmass, just as much as the outside airmass is in motion relative to the air inside the fuselage.
However, the overall dynamics of the flow around the fuselage do produce a net drop in pressure inside the fuselage, if it is unsealed.  There must be locations on the fuselage that would be bad places to put a static port because they are the planes where this pressure drop is being created by the overall dynamics of the flow.
Related answer-- In an unpressurized cabin, why is the cockpit ambient pressure lower than the outside pressure?
Another highly related answer-- Does the pressure at the static ports drop as the aircraft's speed increases?

This answer could be made more complete with additional content but I think it gets at the heart of the matter.

I think my point of confusion is why is the pressure in the cabin (or wherever the alternate static ports are actually located) lower than in the standard static ports that have the same speed of air (I'm assuming) flowing over their openings? Except the "openings" on the alternate would be things like cracks in the door seals and windows. is it because there is more exposed leak area and the pressure drop is cumulative or what?

I think the pressure drop in the cabin is caused by the effect of leaks located in places where the airflow is being forced to accelerate.  It's a good question and it's possible that my answer could be further improved.

I wonder if it doesn't have to do with elongation of the opening in the direction of flow such as the top of the door seal.


text:
How is The Published Runway Touchdown Zone Elevation Decided?
The touchdown zone of a runway can be up to 3,000 feet long if the information I have is accurate.
How is the published Touchdown Zone Elevation determined?  Is it the center, an average of the elevation of the entire touchdown zone, or something else?

Per the AIM it is the highest point in the first 3,000’ of the runway.

@JScarry Looks like an answer, not a comment.


text:
What is the difference in purpose of the 1:40 Obstacle Identification Surface and the 1:50 clearance plane for an ILS glideslope?
I have found references to the Obstacle Identification Surface (OIS), which is approximately a 152ft/NM slope from the end of the runway (1.43 degrees, 1:40 rise/run) and a 1:50 clearance plane noted in the illustration in this answer:

1:50 is 121.5ft/NM, so they clearly aren't one in the same.
Why are there both? What is the difference in purpose?

@ymb1 It's in one of the bubbles in the graphic of the accepted answer.